Amnesty International's call comes months after The Associated Press reported that the UAE and allied militias were running a network of secret detention facilities where torture and abuses were widespread, outside the control of Yemeni President Abed Rabbo Mansour Hadi's government.
In a report titled "God only knows if he's alive," Amnesty said it documented "egregious violations going unchecked, including systemic enforced disappearance and torture and other ill-treatment amounting to war crimes."
The UAE issued a statement Thursday promptly rejecting Amnesty's report, describing it as "politically motivated to undermine" Emirati efforts as part of the Saudi-led coalition in support of the Yemeni government.
In Yemen's 3-year-old civil war, the UAE is part of a Saudi-led coalition battling Iranian-backed Shiite rebels known as Houthis who have taken over most of the country's north. Ostensibly, the Emiratis and Hadi's government are allies in the fight, but tensions between them have been high.
The UAE has built up militias across southern Yemen that government officials say are only loyal to the Emiratis. Those forces have taken over wide swaths of territory in the south, including towns and cities.
Amnesty said that these militias were "created, trained, equipped and financed" by the UAE and are "operating outside the command of their own government." The Emiratis, it added, have also built alliances with Yemeni security officials that bypass the Yemeni government.
The UAE has repeatedly denied it is involved in unlawful detention practices in Yemen. On Sunday, Anwar Gargash, the UAE state minister for foreign affairs, dismissed reports that his country controls prisons as "fake news." Amnesty said the denials come "despite all the evidence to the contrary."
Amnesty said it investigated the cases of 51 men allegedly detained by UAE-backed militias between March 2016 and May 2018 in Aden, Lahj, Abyan, Hadramawt and Shabwa provinces. It said most of the cases involved forced disappearances, and 19 of the men remain missing.
"The families of these detainees find themselves in an endless nightmare where their loved ones have been forcibly disappeared by UAE-backed forces," said Tirana Hassan, crisis response director at Amnesty. "When they demand to know where their loved ones are held, or if they are even still alive, their requests are met with silence or intimidation."
Amnesty said it had documented "widespread use of torture and other ill-treatment in Yemeni and Emirati facilities." Current and former detainees and families gave "horrific accounts of abuse including beatings, use of electric shocks and sexual violence," the watchdog said.
The report quoted an unnamed detainee as saying that he saw a fellow detainee being carried away in a body bag after being repeatedly tortured.
"Ultimately these violations ... should be investigated as war crimes. Both the Yemeni and UAE governments should take immediate steps to end them and provide answers to the families whose husbands, fathers, brothers and sons are missing," Hassan said.
In the AP's investigation last year, former prisoners and security officials described widespread torture at prisons run by the Emiratis and their militias. Thousands of Yemenis swept up in the U.S.-backed campaign against al-Qaida have been held in the prisons without being charged or tried. Some have been interrogated by U.S. personnel inside the facilities, the AP found.
"The UAE does not manage or run prisons in Yemen," said the Emiratis' statement on Thursday. "Prisons in Yemen are under Yemeni authority and fall under the jurisdiction of the country's institutions."
On Monday, Yemeni Interior Minister Ahmed Al-Maysari demanded the UAE shut down or hand over secret prisons. It was the first time he had gone public with the demand in talks with an Emirati official, seeming to contradict the UAE's repeated denials that it has authority over any prisons in Yemen.
Dozens of detainees have been freed from the facilities since the AP did an investigation in June detailing sexual abuse and torture at the sites. Sexual abuses were filmed as a way to turn suspects into informants, detainees reported.
Amnesty's report also urged the UAE's counter-terrorism partners, including the United States, to "take a stand against allegations of torture, including by investigating the role of U.S. personnel in detention-related abuses in Yemen."
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.